The genetic polymorphism of thyroxine-binding globulin (TBG) detected in blacks by isoelectric focusing was studied after 85% deglycosylation or desialylation. Two forms of polymorphism were observed in addition to that existing in the native molecule and caused by differing sialic acid content. These forms, at least one of which is genetically determined, apparently are due to differences in amino acid composition or amidation. A 27,000 molecular weight protein (27 K Protein) has been detected in TBG preparations from human plasma. This protein appears to be immunologically related to TBG and binds T4, but contains little or no carbohydrate.